This invention relates to a method of displaying characters on a cathode ray tube (CRT) screen, and more particularly to a method and apparatus for smoothing the edges of characters thus displayed.
The inside face of a CRT screen is coated with phosphor, or some similar material, that glows when struck by an electron beam. A complete image is assembled on the CRT by "scanning" the screen with such an electron beam. This scanning process may be likened to the manner in which a person reads a page wherein his eyes read one line from left to right, and then drops down one line and read again from left to right. The electron beam inside a CRT scans the screen in much the same way in that the beam moves along horizontal scan lines. When its movement across one horizontal scan line is complete, it drops down to the next horizontal scan line and sweeps across it. In this fashion, the beam scans across the entire face of the CRT screen by sequentially traversing each of the horizontal scan lines that, positioned one below the other, fill the CRT screen.
The electron beam causes a glow or light to appear when it strikes the phosphor coating on the inside of the screen, which glow or light remains present for a short time after the phosphor is struck. Likewise, when the beam is interrupted, the phosphor coating will not glow and the screen appears dark. Because the electron beam moves across the entire screen many times in a fraction of a second, a complete image can be assembled thereon by selectively interrupting this beam as it moves across the various horizontal scan lines.
There are principally three conical signals associated with this image assembling process that must be generated externally to the CRT. First, a horizontal timing signal is needed to move the electron beam at a constant velocity across the horizontal scan lines. Second, a vertical timing signal is required to position the electron beam vertically at the beginning of a new horizontal scan line after an adjacent horizontal scan has been completed. Third, a control signal is needed to selectively interrupt the electron beam as it moves along each horizontal scan line so that only selected areas of the CRT screen will glow, thus creating the desired image. Because the area where the image appears on the CRT screen is commonly termed the "raster", this third signal is often called the "raster control signal", and will be so referred to hereinafter. The invention disclosed herein is concerned with a method and apparatus of timing and shaping the raster control signal when the image to be displayed on the CRT screen is a character (letter, number, or symbol) so as to smooth the edges thereof.
The drawing of FIG. 1 illustrates how the image of a character is assembled on a CRT screen, and will be helpful in explaining the state of the prior art. Shown in the upper portion of FIG. 1 is the image of the character of letter "W" as it might appear on a CRT screen. An electron beam scans across the face of the CRT from left to right along horizontal scan lines 101, starting with scan line 0. At the completion of scan line 0, the electron beam returns to the left and simultaneously drops down one line to begin its left to right movement along scan line 1. Thus, the path of the electron beam follows a zig-zag path down the face of CRT as represented by line A--A.
As the electron beam moves along each horizontal scan line, raster control signals 103, shown in the lower portion of FIG. 1, are generated to selectively inhibit the electron beam from striking the CRT phosphor coated screen, thereby creating selective dark areas thereon. These raster control signals 103, one corresponding to each horizontal scan line 101, are represented in FIG. 1 as a low signal when the electron beam is to be off (inhibited, or dark screen), and a high signal when it is to be on (not inhibited, or a light screen). Thus, the raster control signal corresponding to horizontal scan line zero (0) is a low signal for the entire time it takes the electron beam to move horizontally across the scan line because the CRT screen is to remain dark for the entire length thereof. In contrast, the raster control signal corresponding to horizontal scan line 1 is switched to a high signal at time t.sub.1, which time corresponds to point p.sub.1 on the CRT screen, and is likewise switched to a low signal at time t.sub.2, corresponding to point p.sub.2. This process causes the screen to appear light between points p.sub.1 and p.sub.2, thus creating the upper lefthand edge of the character "W". Likewise, the raster control signal is switched from low to high at time t.sub.3 of its horizontal sweep across scan line 1, and from high to low at time t.sub.4, in order to cause the upper righthand edge of the "W" to appear on the CRT screen. In a similar fashion, the raster control signal is used to selectively switch the electron beam "on" and "off" ("not inhibited" or "inhibited", respectively) as it scans along each horizontal scan line, thereby causing the screen to become light only during those selective portions of each scan line that constitute segments of the character "W". (Not shown in FIG. 1 are the blanking signals used to keep the electron beam inhibited during its diagonal return from the end of one scan line to the beginning of another.)
As the prior art drawing of FIG. 1 illustrates, a character displayed by the process described above will have "jagged" or "stair-stepped" edges. This is because the raster control signals are typically generated using digital techniques, i.e., employing a memory to store information defining which segments of each horizontal scan line are to appear light for each of the possible characters that can be displayed. Such techniques, by their very nature, segment each horizontal scan line into bits or fragments that must either be light or dark. The width of these "bits" is termed the character resolution time, and is illustrated in FIG. 1 as time "T.sub.c ". Digital techniques also cause the raster control signal to turn on and off sharply, even when a character edge is oblique to vertical. Such edges appear "stair-stepped" even when T.sub.c is diminished toward zero.
The prior art employs two basic methods to smooth the "jagged" or "stair-stepped" edges that are inherent in characters displayed on CRT screens by the method described above. The first is simply to decrease the character resolution time, T.sub.c. The prior art drawing of FIG. 2 illustrates the effect that a reduction in T.sub.c can have. Theoretically, of course, T.sub.c can be made smaller and smaller until the human eye can no longer perceive the jagged edges appearing on the sloping edges of the character. This first approach yields excellent results for steep sloping edges, but the method of making T.sub.c very small is very expensive to implement. This added expense results primarily from the increased memory, and corresponding increased amount of circuitry, that the smaller-character-resolution-time approach requires in order to "remember" what each smaller segment of each horizontal scan line is supposed to be--light or dark. Moreover, unless the character resolution time is made extremely small, the undesirable stair-step edges remain, only on a smaller scale. Also, unless the density of horizontal scan lines is greatly increased, the jaggedness of slopes oblique to vertical is not diminished by reducing T.sub.c.
The second method used in the prior art to smooth the inherent jagged edges of CRT displayed characters is to pass the raster control signal through a low pass filter prior to allowing it to control the electron beam. The low pass filter, of course, removes all the high frequencies from the signal, thus rounding out its corners and decreasing its rise and fall times. This low pass filter approach is simple to implement, but unfortunately the absence of the high frequencies prevents the raster control signal from sharply turning the electron beam on or off. The result is a smudged look, i.e., the character appears as though it is out of focus.